The present invention relates to a toner, composed of a plurality of mother particles and a plurality of external additive particles such as particles of silica, for developing an electrostatic latent image formed on a latent image carrier, and to an image forming apparatus for forming an image using the toner.
More particularly, the present invention relates to a toner, in which at least a plurality of particles of silica are used as an external additive and which has a function of preventing filming of toner to toner-contact members such as a latent image carrier, a toner carrier, and a toner regulating member, with which the toner may come in contact, thereby improving the durability of toner-contact members and also allowing the low-temperature fixing to be conducted well.
Further, the present invention relates to a toner in which at least a plurality of particles of titanium oxide or particles of alumina are used as an external additive and which is capable of stabilizing the charging property of toner, and to an image forming apparatus using this toner.
In an image forming apparatus using a toner, an image is obtained by developing an electrostatic latent image on a latent image carrier by a toner, transferring the developed image from the latent image carrier to a recording media such as paper to form a transferred image of the electrostatic latent image exposed on the latent image carrier, and finally fixing the transferred image to the recording media.
Among conventional image forming apparatuses, there is a multicolor image forming apparatus of intermediate transfer type as shown in FIG. 1. In an image forming apparatus 1 of this type, an image is exposed to light as an electrostatic latent image onto a photoreceptor 2 as a latent image carrier. The electrostatic latent image on the photoreceptor 2 is developed by yellow, magenta, cyan, and black non-magnetic mono-component developing devices 3, 4, 5, 6 in this order (the order of respective colors is arbitrary) so as to obtain visible developed images. The developed images on the photoreceptor 2 are superposed and toned on an intermediate transfer belt 7a of an intermediate transfer member 7, thus achieving primary transfer. After the primary transfer, the toned image is transferred to a recording media 9 such as a paper on a secondary transfer roller 8a of the transferring device 8, thus achieving secondary transfer. After that, the image is heated and fixed to the recording media 9 by a fixing device 10, thereby obtaining a desired image on the recording media 9.
Then, residual toner particles T′ remaining on the photoreceptor 2 after the developed images are transferred to the intermediate transfer belt 7a are removed by a cleaning blade 11 and are collected in a residual toner box 12. Similarly, residual toner particles remaining on the intermediate transfer belt 7a after the primary transferred image is transferred to the recording media 9 are removed by a cleaning blade and are collected in a residual toner box.
The respective non-magnetic mono-component developing devices 3, 4, 5, 6 have substantially the same structure. That is, each of the developing devices 3, 4, 5, 6 is of a contact developing type that a conductive developing roller 16 is arranged in contact with the photoreceptor 2. As shown in FIG. 2(a), toner particles T in a toner container 13 are carried by a toner carrying means 14 to a toner supply roller 15 as toner supply means and is further supplied to the developing roller 16 by the toner supply roller 15. Accordingly, the toner particles T are held on the surface of the developing roller 16. A bias voltage composed of an alternating current superimposed on a direct current is applied to the developing roller 16 and the developing roller 16 is rotated at a high speed, whereby the toner particles T are regulated to be in a uniform thin layer by a toner regulating member 17 as toner regulating means, which is in press contact with the surface of the developing roller 16, and are uniformly charged. After that, the toner particles T on the developing roller 16 are uniformly conveyed toward the photoreceptor 2 which is in contact with the developing roller 16.
The toner particles T on the developing roller 16 is transferred to the photoreceptor 2 by developing voltage applied to the developing roller 16, whereby an electrostatic latent image is developed with the toner particles T on the photoreceptor 2.
The non-magnetic mono-component developing method employing conductive developing roller 16 also includes a non-contact developing method in which the developing roller 16 and a photoreceptor 2 are spaced apart from each other. In developing of the non-contact developing method, a developing voltage is applied to the developing roller 16 and toner particles T on the developing roller 16 is transferred by jumping to the photoreceptor 2 due to the developing voltage. That is, an electrostatic latent image on the photoreceptor 2 is developed with the toner particles T by jumping-developing.
In the conventional non-magnetic mono-component developing device, some toner particles T may be fused to toner-contact members, with which the toner may come in contact, such as the developing roller 16, the toner regulating blade 17, and/or the photoreceptor 2 or, alternatively, liberated external additive particles may be separated from the toner particles, thus leading to the occurrence of filming. Due to the filming, the insufficiency of toner carrying function of the developing roller 16, the insufficiency of regulating function of the toner regulating blade 17, and/or the insufficiency of developing function of the photoreceptor 2 may be occurred, thus further leading to degradation of image quality.
When the toner particles T are not uniformly charged, the insufficiency of toner carrying function of the developing roller 16, and/or the insufficiency of developing function of the photoreceptor 2 may be similarly occurred, thus further leading to similar problems.
To solve these problems, conventionally, resin mother particles 18 of the toner T are coated with external additive particles 19 as shown in FIG. 2(b). The size of the external additive particles 19 is set to be significantly smaller than the size of the mother particles 18.
Since use of a fixing device 10 for heat fixing enables the low-temperature fixing, conventionally, the mother particles 18 of the toner T have a low softening point so that the mother particles 18 are soft even at ordinary temperature.
On the other hand, in the conventional non-magnetic mono-component developing device, at least silica (SiO2) 19 is added as the external additive 19 in order to prevent the occurrence of the filming.
Further, in the non-magnetic mono-component developing device, titanium oxide (TiO2) 19 is added as the external additive 19 serving as a charge controlling agent as shown in FIG. 2(b) in order to stabilize the charge of the toner T.
Furthermore, in the non-magnetic mono-component developing device, alumina (Al2O3) 19 is added as the external additive 19 serving as a charge controlling agent and a cleaner as shown in FIG. 2(b) in order to stabilize the charge of the toner T and to clean the photoreceptor 2.
Particularly, most of conventional color toners employ a combination of silica and titanium oxide.
By the way, as for the aforementioned toner T, the external additive particles 19 adhere to the mother particles 18 by agitating the mother particles 18 and the external additive particles 19. As shown in FIG. 2(b), actually there are mother particles 18 and external additive particles 19 which adhere to each other (it should be noted that the numeral 18 is used for designating mother particles themselves as described above and also designating mother particles to which external additive particles 19 adhere and that the numeral 19 is used for designating external additive particles themselves as described above and also designating external additive particles adhering to mother particles), liberated mother particles 18′ to which no external additive particle 19 adheres, and liberated external additive particles 19′ adhering no mother particle 18 and they exist in the mixed state.
However, particularly for mother particles 18 having a low softening point, since the mother particles 18 are relatively soft, when the rate of the liberated mother particles 18′ in the toner T are increased, the liberated mother particles 18′ are fused to the aforementioned toner-contact members, thus facilitating the occurrence of filming and thus reducing the durability of the toner-contact members.
It may be considered to increase the adding amount of the external additive particles 19 to reduce the amount of the liberated mother particles 18′. When silica (SiO2) 19 is employed as the external additives, the increase in the amount of silica particles 19 facilitates silica particles 19 to enter in the mother particles 18 because the mother particles 18 are relatively soft. When the silica particles 19 enter in the mother particles 18, the effect of the silica particles 19 coating the mother particles 18 is reduced. Therefore, the toner particles T are fused onto the toner-contact members, thus facilitating the occurrence of filming and thus reducing the durability of the toner-contact members in the same manner as mentioned above.
In addition, as the adding amount of silica particles 19 is increased, the amount of liberated silica particles 19′ as the liberated external additives 19′ is also increased. Therefore, the liberated silica particles 19′ may adhere to the toner-contact members, thus also leading to the occurrence of filming.
Moreover, as the amount of silica particles 19 is increased, not only a large quantity of heat for heat fixing is consumed by the silica particles 19, but also heat is hard to be transferred to the mother particles 18 because the mother particles 18 are coated with many silica particles 19. As a result, it is easy to be subjected to deterioration of low-temperature fixing property. In addition, the coating on the mother particles 18 by the silica particles 19 blocks the elution of releasing agent, thus leading to the deterioration of fixing property.
When titanium oxide 19 is employed as the external additive 19 in the toner T (it should be noted that the numeral 18 is used for designating mother particles themselves as described above and also designating mother particles to which titanium oxide particles 19 adhere and that the numeral 19 is used for designating titanium oxide particles themselves as described above and also designating titanium oxide particles adhering to mother particles), the titanium oxide particles 19 adhere to the mother particles 18 by mixing or agitating the mother particles 18 and the titanium oxide particles 19. As shown in FIG. 2(b), actually there are mother particles 18 and titanium oxide particles 19 which adhere to each other, liberated mother particles 18′ to which no titanium oxide particle 19 adheres, and liberated titanium oxide particles 19′ adhering no mother particle 18 and they exist in the mixed state.
However, the increase in the amount of liberated mother particles 18′ leads to excessive charge due to liberated mother particles 18′, thus producing the insufficiency of development.
It may be considered to increase the adding amount of the titanium oxide particles 19 to reduce the amount of the liberated mother particles 18′. However, the increase in the amount of titanium oxide particles 19 leads to insufficient charge, thus bringing about image defects and toner scattering.
In addition, as the adding amount of titanium oxide particles 19 is increased, the amount of liberated titanium oxide particles 19′ is also increased. Therefore, the liberated titanium oxide particles 19′ may adhere to the toner-contact members, thus facilitating the occurrence of filming.
When alumina 19 is employed as the external additive 19 in the toner T (it should be noted that the numeral 18 is used for designating mother particles themselves as described above and also designating mother particles to which alumina particles 19 adhere and that the numeral 19 is used for designating alumina particles themselves as described above and also designating alumina particles adhering to mother particles), the alumina particles 19 adhere to the mother particles 18 by mixing or agitating the mother particles 18 and the alumina particles 19. As shown in FIG. 2(b), actually there are mother particles 18 and alumina particles 19 which adhere to each other, liberated mother particles 18′ to which no alumina particle 19 adheres, and liberated alumina particles 19′ adhering no mother particle 18 and they exist in the mixed state.
However, as mentioned above, the increase in the amount of liberated mother particles 18′ in the toner T leads to excessive charge due to liberated mother particles 18′, thus producing the insufficiency of development. For this, it may be considered to increase the adding amount of the alumina particles 19 to reduce the amount of the liberated mother particles 18′. However, since the increase in the amount of alumina particles 19 increases the amount of liberated alumina particles 19′, the liberated alumina particles 19′ adhere to the toner-contact members, thus facilitating the occurrence of filming.
In addition, since the alumina 19 has a function of controlling the charging property of the toner, the increase in the amount of alumina particles 19 deteriorates the charging property of the toner.
On the other hand, conventional toners include polymerized toners which are prepared by a polymerization method and pulverized toners which are prepared by a pulverization method and are rounded by heat treatment. Mother particles of these toners have relatively large roundness of 0.95 or more and are approximately formed in spheres. In the mother particles 18 having a low softening point particularly, the larger the roundness of a mother particle 18 is, the more spherical the mother particle 18 is. In this state, the mother particles 18 of the residual toner T′ remaining on the photoreceptor 2 or the intermediate transfer belt 7a after transferred may pass through a space between the cleaning blade 11 and the photoreceptor 2 or between the cleaning blade 11 and the intermediate transfer belt 7a, that is, may not be removed by the cleaning blade 11, thus deteriorating the cleaning property of the cleaning blade 11. As a result of this, these particles remain adhering to the photoreceptor 2 and/or the intermediate transfer belt 7a, thus leading to the occurrence of filming. This reduces the durability of the photoreceptor 2 and the intermediate transfer belt 7a as the toner-contact members.
As the amount of liberated external additive particles 19′ in the toner T is increased, it is difficult to effectively coat the mother particles 18 with the external additive particles 19. Accordingly, as mentioned above, the mother particles 18 are easily fused on the toner-contact members, thus facilitating the occurrence of filming. In addition, liberated external additives 19′ easily adhere to the toner-contact members because the liberated external additive particles 19′ are many, thereby also facilitating the occurrence of filming and thus decreasing the durability of the toner-contact members.
Moreover, many liberated external additive particles 19′ easily soil the inside of each developing device 3, 4, 5, 6. Particularly, once charging members, such as the developing roller 16 and the toner regulating blade 17, for charging the toner T, are coated with the liberated external particles 19′, it is hard to charge the toner T, thereby reducing the charging property of the toner T.
When the amount of liberated external additive particles 19′ is small, not only a large quantity of heat for heat fixing is consumed by the external additive particles 19, but also heat is hard to be transferred to the mother particles 18 because the mother particles 18 are coated with many external additive particles 19. As a result, it is easy to be subjected to deterioration of low-temperature fixing property and the elution of releasing agent is blocked, thus leading to the deterioration of fixing property.
There is another problem that external additive particles may enter in mother particles so that components having low melting point of the mother particles soil the toner-contact members.
When the amount of liberated external additive particles 19′ is small, the liberated external additive particles 19′ can not exhibit the effect of cleaning the toner-contact members such as the photoreceptor 2 and the intermediate transfer belt 7a. 
Generally, there are a large number of mother particles 18 of which particle diameter is about 1.5 μm or more. The smaller the particle diameter is, the lower the roundness of the mother particle 18 is. This is due to some problems on production of toners T such as in the pulverization method. With lower roundness, the mother particle 18 may have not only increased portions (e.g. sharp edges) to which the external additive particles 19 hardly adhere but also concaved portions in which the external additive particles 19 may enter, thereby hardly exhibiting the effect of the external additive particles 19.
As the amount of liberated mother particles 18′ in the toner T is increased, the liberated mother particles 18′ easily adhere to the toner-contact members. In addition, since the number of external additive particles 19 adhering to the mother particles 18 is reduced, the mother particles 18 not the liberated mother particles 18′ also easily adhere to the toner-contact members. As the mother particles 18 adhere to the toner-contact members, filming occurs on the toner-contact members, thus reducing the durability of the toner-contact members. The mother particles 18 having a low softening point especially easily adhere to the toner-contact members because of the mother particles 18 are relatively soft, thus facilitating the occurrence of filming on the toner-contact members. In addition, since micro particles as mother particles 18 adhering to the toner-contact members are further hardly removed, the possibility of occurrence of filming on the toner-contact members is increased.
When titanium oxide particles adhere to the mother particles 18 strongly in comparison to silica particles, the toner can not be charged uniformly, thus deteriorating the uniformity of charging. The reason why the uniformity of charging is deteriorated may be that the titanium oxide particles hardly move in comparison to the silica particles.